1. Field of the Invention
The present invention relates to an electronic endoscope apparatus for obtaining a color image of a target observation area by irradiating white illumination light thereon. More specifically, the invention relates to an electronic endoscope apparatus used in combination with a near infrared light irradiation unit that irradiates near infrared light on a target irradiation section. The invention also relates to a program for use with the electronic endoscope apparatus for generating a color image signal.
2. Description of the Related Art
Developments of optical tomographic image obtaining systems for obtaining optical tomographic images of living bodies and the like are under way. Well-known methods for obtaining optical tomographic images include the method using optical interference caused by frequency-swept coherent light, the method using optical interference caused by low coherence light, and the like.
In particular, an OCT (Optical Coherence Tomography) system that obtains an optical tomographic image by measuring a light intensity of low coherence interference light through heterodyne detection is put into practical use.
The OCT system obtains optical tomography information of a target measuring object through the steps of: splitting low coherence light outputted from a light source, which includes SLD (Super Luminescent Diode) or the like, into measuring light and reference light; slightly shifting the frequency of either the reference light or measuring light by a piezo device or the like; irradiating the measuring light on the target measuring object; causing interference between the reflected light reflected from the target irradiation section and the reference light; and measuring the optical intensity of the interference light through heterodyne detection as described, for example, in non-patent document “Real-time in vivo imaging of human gastrointestinal ultrastructure by use of endoscopic optical coherence tomography with a novel efficient interferometer design”, A. M. Rollins et al., OPTICS LETTERS, Vol. 24, No. 19, pp. 1358-1360, 1999. By slightly changing the optical path length of the reference light through slight movement of a movable mirror or the like disposed in the optical path of the reference light, information at a predetermined depth of the target irradiation section where the optical path length of the reference light corresponds to the optical path length of the measuring light. Then, by repeating the measurement while slightly moving the section for irradiating the measuring light, an optical tomographic image of a predetermined area may be obtained.
The use of such OCT systems allow diagnosis of invasion depth of early cancer and the like, so that efforts for developing a method for obtaining an optical tomographic image within a body cavity by guiding measuring light and reflected light through the OCT probe insertable in a forceps channel of an endoscope apparatus are underway as described for example, in U.S. Pat. No. 6,668,185. The aforementioned patent publication describes an OCT system that includes an OCT probe having an optical fiber for guiding measuring light, and a mirror disposed at the distal end of the optical fiber and reflects the measuring light to an orthogonal direction. The OCT system displays a tomographic image of a wall of a body cavity by inserting the OCT probe into the body cavity through the forceps channel of the endoscope, and rotating the mirror disposed at the distal end of the probe.
Most of such OCT systems use near infrared light as the measuring light in order to reduce the optical loss at the target irradiation section. The near infrared light, however, is not visible, so that visible light, such as He—Ne laser, or the like is concentrically superimposed thereon and used as aiming light.
In the mean time, in the field of laser therapy systems, near infrared light, such as YAG laser or the like, is also used, and here again visible light is used as the aiming light.
Recently, in putting into practical use, the downsizing and simplified structure have been demanded for the near infrared light irradiation unit used in optical tomographic image obtaining systems and the like. In order to superimpose the aiming light concentrically with the near infrared light, however, it is necessary to dispose an optical device, such as a dichroic mirror or the like, or a coupler for combining the light or the like in the optical path. This causes a problem that the downsizing and simplified structure for the near infrared irradiation unit is prevented. On the other hand, the near infrared light irradiation unit without aiming light causes a problem that the section irradiated by the near infrared light is not visually recognized.